coatings Investigation of the Properties of Ti-TiN-(Ti,Cr,Mo,Al)N Multilayered Composite Coating with Wear-Resistant Layer of Nanolayer Structure

: The article describes the results of an investigation focused on the properties of the Ti-TiN-(Ti,Cr,Mo,Al)N multilayered composite coating with a wear-resistant layer of nanolayer structure. A transmission electron microscope was used to study the coating structure. The examination of the phase composition using selected area di ﬀ raction electron pattern has detected the presence of two phases, including c-(Ti,Cr,Mo,Al)N and h-AlN. The cutting properties of the tool with the coating under consideration were studied during the turning of AISI 1045 steel at v c = 300 m / min, f = 0.25 mm / rev, and a p = 1.0 mm. After 16 min of cutting, the wear rate for the tool with the Ti-TiN-(Ti,Cr,Mo,Al)N coating was 1.9 times lower compared to the wear rate for the tool with the (Ti,Al)N commercial monolithic coating. As a result of the investigation focused on the fracture pattern on the coating during the cutting, the brittle nature of the fracture has been detected with a noticeable e ﬀ ect of adhesive fatigue mechanisms.


Introduction
Modified coatings developed for various purposes, particularly coatings for metal-cutting tools, are being actively implemented in various areas of manufacturing activity. At the same time, the trends of modern manufacturing suggest toughening requirements for coatings. In particular, an increase in the cutting speed leads to an increase in temperature in the cutting zone, and, accordingly, the crucial feature of the coatings is heat resistance [1][2][3][4][5]. With an increase in temperature, oxidation and diffusion processes become more active and intensify tool wear [6][7][8][9][10]. Accordingly, one of the significant factors is the ability of a coated tool to resist the oxidation and diffusion wear. The coatings of traditional composition, such as TiN, TiC, ZrN, CrN, or (Ti,Al)N, can no longer meet the requirements of the modern manufacturing, and new coatings with enhanced properties are required. One of the ways to The nanoindentation technique and an Instron Wilson Hardness Group Tukon tester at the load of 0.01 N were used to determine the coating microhardness.
During the turning of workpieces made of AISI 1045 steel, a CU 500 MRD lathe (ZMM Sliven, Sliven, Bulgaria) with a ZMM CU500 MRD variable-speed drive (ZMM Bulgaria, Sofia, Bulgaria) was applied. No coolants or lubricants were used during the process of cutting. SNUN ISO 1832:2012 carbide inserts played a role of substrates, with the parameters as follows: γ = -7 • , α = 7 • , λ = 0, r = 0.4 mm; cutting mode: f = 0.25 rpm, a p = 1.0 mm, and v c = 300 m/min. Four experiments were conducted for each coating, and the obtained values of flank wear were processed to get the polynomial functions exhibited on the curve. The limit wear criterion was assumed as flank wear rate VB max = 0.4 mm. Five tests of cutting properties were carried out, after which the information was statistically processed. Average values were determined for five experiments, these average values were used to plot the graph. Polynomial dependencies were obtained, on the basis of which graphs of the dependence of the flank wear on the cutting time were plotted.

Study of the Chemical Composition and Nanostructure of the Ti-TiN-(Ti,Cr,Mo,Al)N Coating
According to the results of 20 conducted measurements, the average hardness of the coating was 42 ± 1.3 GPa, which is fairly high for nitride coatings. The coating structure includes an adhesion layer of Ti with the thickness of about 50 nm, a transition layer of TiN with the thickness of about 600 nm, and a wear-resistant layer of (Ti,Cr,Mo,Al)N with the thickness of about 2700 nm ( Figure 1). The thickness of the functional layers of the coating was selected based on their optimal ratio [32,35]. The wear-resistant layer of (Ti,Cr,Mo,Al)N is formed by a 22-nanolayer period with λ of about 120 nm [11,33]. The nanostructures of the studied coating Ti-TiN-(Ti,Cr,Mo,Al)N in comparison with the (Ti,Al)N monolayer coating are presented in Figure 2. Figure 2a illustrates that the nanostructure of Ti-TiN-(Ti,Cr,Mo,Al)N coating includes nanolayers with the high content of Al (lighter bands) and nanolayers with the high content of Cr-Mo and Ti (darker bands). For Ti-TiN-(Ti,Cr,Mo,Al)N coating, the value of nanolayer period λ [11,33] is about 120 nm, and the thicknesses of nanolayers are within a range of 1-8 nm. The results of the studies of the coatings phase compositions using the Selected Area Electron Diffraction (SAED) method are presented in Figure 2c,d. The nanostructures of the studied coating Ti-TiN-(Ti,Cr,Mo,Al)N in comparison with the (Ti,Al)N monolayer coating are presented in Figure 2. Figure 2a illustrates that the nanostructure of Ti-TiN-(Ti,Cr,Mo,Al)N coating includes nanolayers with the high content of Al (lighter bands) and nanolayers with the high content of Cr-Mo and Ti (darker bands). For Ti-TiN-(Ti,Cr,Mo,Al)N coating, the value of nanolayer period λ [11,33] is about 120 nm, and the thicknesses of nanolayers are within a range of 1-8 nm. The results of the studies of the coatings phase compositions using the Selected Area Electron Diffraction (SAED) method are presented in Figure 2c  The nanostructures of the studied coating Ti-TiN-(Ti,Cr,Mo,Al)N in comparison with the (Ti,Al)N monolayer coating are presented in Figure 2. Figure 2a illustrates that the nanostructure of Ti-TiN-(Ti,Cr,Mo,Al)N coating includes nanolayers with the high content of Al (lighter bands) and nanolayers with the high content of Cr-Mo and Ti (darker bands). For Ti-TiN-(Ti,Cr,Mo,Al)N coating, the value of nanolayer period λ [11,33] is about 120 nm, and the thicknesses of nanolayers are within a range of 1-8 nm. The results of the studies of the coatings phase compositions using the Selected Area Electron Diffraction (SAED) method are presented in Figure 2c,d. The study of the nature of the distribution of elements in the nanolayer periods ( Figure 3b) finds that the content of each element changes significantly within a nanolayer period. In particular, the content of Ti changes from 7 to 60 at.%, and the content of Al-from 3 to 27 at.%. The above ensures a smooth, gradient transition from harder and more wear-resistant layers with the high Al content to more ductile layers with the low Al content.  Let us consider the influence of the nanolayer structure of the coating on its crystalline structure. Earlier, it has been found that the nanolayer structure affects the grain sizes by reducing them [11,33]. At the same time, the grain size of the coating is not always limited by the boundaries of a nanolayer or a nanolayer period [11]. The Ti-TiN-(Ti,Cr,Mo,Al)N coating under study demonstrates columnar The study of the nature of the distribution of elements in the nanolayer periods ( Figure 3b) finds that the content of each element changes significantly within a nanolayer period. In particular, the content of Ti changes from 7 to 60 at.%, and the content of Al-from 3 to 27 at.%. The above ensures a smooth, gradient transition from harder and more wear-resistant layers with the high Al content to more ductile layers with the low Al content. Let us consider the influence of the nanolayer structure of the coating on its crystalline structure. Earlier, it has been found that the nanolayer structure affects the grain sizes by reducing them [11,33]. At the same time, the grain size of the coating is not always limited by the boundaries of a nanolayer or a nanolayer period [11]. The Ti-TiN-(Ti,Cr,Mo,Al)N coating under study demonstrates columnar Ti-22 at.%, Cr-38 at.%, Al-11 at.%, Mo-10 at.%. The study of the nature of the distribution of elements in the nanolayer periods (Figure 3b) finds that the content of each element changes significantly within a nanolayer period. In particular, the content of Ti changes from 7 to 60 at.%, and the content of Al-from 3 to 27 at.%. The above ensures a smooth, gradient transition from harder and more wear-resistant layers with the high Al content to more ductile layers with the low Al content.
Let us consider the influence of the nanolayer structure of the coating on its crystalline structure. Earlier, it has been found that the nanolayer structure affects the grain sizes by reducing them [11,33]. At the same time, the grain size of the coating is not always limited by the boundaries of a nanolayer or a nanolayer period [11]. The Ti-TiN-(Ti,Cr,Mo,Al)N coating under study demonstrates columnar crystals with sizes noticeably larger than the value of nanolayer period (Figure 4a), and the grain structure of the coating can be seen more clearly in the reverse contrast image (Figure 4b). The nanolayer structure of this coating does not stop the growth of crystals (Figure 4c). However, as noted earlier [11,33], the presence of the nanolayer structure allows the formation of crystals with significantly smaller sizes than crystals in coatings with monolayer structures. A comparison of SAED patterns on the Ti-TiN-(Ti,Cr,Mo,Al)N nanolayer coating (Figure 2c) and on the (Ti,Al)N monolithic coating (Figure 2d) demonstrates the significantly smaller size of crystals in the Ti-TiN-(Ti,Cr,Mo,Al)N coating.
Coatings 2020, 10, x FOR PEER REVIEW 6 of 13 crystals with sizes noticeably larger than the value of nanolayer period (Figure 4a), and the grain structure of the coating can be seen more clearly in the reverse contrast image (Figure 4b). The nanolayer structure of this coating does not stop the growth of crystals (Figure 4c). However, as noted earlier [11,33], the presence of the nanolayer structure allows the formation of crystals with significantly smaller sizes than crystals in coatings with monolayer structures.

Study of the Cutting Properties and the Wear Pattern on Tools with the Ti-TiN-(Ti,Cr,Mo,Al)N Coating
The studies of the cutting properties of the carbide tools found that the use of Ti-TiN-(Ti,Cr,Mo,Al)N coating can significantly reduce the flank wear compared to the tools with the (Ti,Al)N commercial coating. Figure 6 illustrates that after 7 min of operation, the wear of the carbide inserts with the (Ti,Al)N coating increases sharply, while the inserts with Ti-TiN-(Ti,Cr,Mo,Al)N coating demonstrate much lower wear, and the wear rate decreases.
The good wear resistance of the Ti-TiN-(Ti,Cr,Mo,Al)N coating can be explained by the formation of tribological oxide films of MoO3 and Cr2O3, which favourably transform the cutting conditions [42][43][44][45][46][47]. The investigation of wear areas on the rake face of the carbide inserts after 16 min of operation ( Figure 7) found that the tool with coating (Ti,Al)N demonstrated the higher rake wear, which manifested itself in the formation of a crater and a notch wear compared to the tool with Ti-TiN-(Ti,Cr,Mo,Al)N coating.

Study of the Cutting Properties and the Wear Pattern on Tools with the Ti-TiN-(Ti,Cr,Mo,Al)N Coating
The studies of the cutting properties of the carbide tools found that the use of Ti-TiN-(Ti,Cr,Mo,Al)N coating can significantly reduce the flank wear compared to the tools with the (Ti,Al)N commercial coating. Figure 6 illustrates that after 7 min of operation, the wear of the carbide inserts with the (Ti,Al)N coating increases sharply, while the inserts with Ti-TiN-(Ti,Cr,Mo,Al)N coating demonstrate much lower wear, and the wear rate decreases.

Study of the Cutting Properties and the Wear Pattern on Tools with the Ti-TiN-(Ti,Cr,Mo,Al)N Coating
The studies of the cutting properties of the carbide tools found that the use of Ti-TiN-(Ti,Cr,Mo,Al)N coating can significantly reduce the flank wear compared to the tools with the (Ti,Al)N commercial coating. Figure 6 illustrates that after 7 min of operation, the wear of the carbide inserts with the (Ti,Al)N coating increases sharply, while the inserts with Ti-TiN-(Ti,Cr,Mo,Al)N coating demonstrate much lower wear, and the wear rate decreases.
The good wear resistance of the Ti-TiN-(Ti,Cr,Mo,Al)N coating can be explained by the formation of tribological oxide films of MoO3 and Cr2O3, which favourably transform the cutting conditions [42][43][44][45][46][47]. The investigation of wear areas on the rake face of the carbide inserts after 16 min of operation (Figure 7) found that the tool with coating (Ti,Al)N demonstrated the higher rake wear, which manifested itself in the formation of a crater and a notch wear compared to the tool with Ti-TiN-(Ti,Cr,Mo,Al)N coating. The good wear resistance of the Ti-TiN-(Ti,Cr,Mo,Al)N coating can be explained by the formation of tribological oxide films of MoO 3 and Cr 2 O 3 , which favourably transform the cutting conditions [42][43][44][45][46][47].
The investigation of wear areas on the rake face of the carbide inserts after 16 min of operation (Figure 7) found that the tool with coating (Ti,Al)N demonstrated the higher rake wear, which manifested itself in the formation of a crater and a notch wear compared to the tool with Ti-TiN-(Ti,Cr,Mo,Al)N coating. The study of the fracture pattern finds that, given the noticeable microdeformations in the structure of the carbide substrate, Ti-TiN-(Ti,Cr,Mo,Al)N coatings are characterised by sufficient ductility and resistance to brittle fracture. The study of the fracture patterns on the Ti-TiN-(Ti,Cr,Mo,Al)N coatings on the rake faces of the tools (Figure 9) finds that the fracture process is accompanied by active cracking. The Ti-TiN-(Ti,Cr,Mo,Al)N coating exhibits both inclined and transverse cracks (Figure 9). On the Ti-TiN-(Ti,Cr,Mo,Al)N coating, the brittle fracture accompanied by chipping of some fragments is typical (Figure 9b-e). Figure 9a illustrates that the wear surface of the coating on the boundary of its fracture is quite smooth and there are signs of local chipping of coating fragments.  The study of the fracture pattern finds that, given the noticeable microdeformations in the structure of the carbide substrate, Ti-TiN-(Ti,Cr,Mo,Al)N coatings are characterised by sufficient ductility and resistance to brittle fracture. The study of the fracture patterns on the Ti-TiN-(Ti,Cr,Mo,Al)N coatings on the rake faces of the tools (Figure 9) finds that the fracture process is accompanied by active cracking. The Ti-TiN-(Ti,Cr,Mo,Al)N coating exhibits both inclined and transverse cracks (Figure 9). On the Ti-TiN-(Ti,Cr,Mo,Al)N coating, the brittle fracture accompanied by chipping of some fragments is typical (Figure 9b-e). Figure 9a illustrates that the wear surface of the coating on the boundary of its fracture is quite smooth and there are signs of local chipping of coating fragments. The study of the fracture pattern finds that, given the noticeable microdeformations in the structure of the carbide substrate, Ti-TiN-(Ti,Cr,Mo,Al)N coatings are characterised by sufficient ductility and resistance to brittle fracture.
The study of the fracture patterns on the Ti-TiN-(Ti,Cr,Mo,Al)N coatings on the rake faces of the tools (Figure 9) finds that the fracture process is accompanied by active cracking. The Ti-TiN-(Ti,Cr,Mo,Al)N coating exhibits both inclined and transverse cracks (Figure 9). On the Ti-TiN-(Ti,Cr,Mo,Al)N coating, the brittle fracture accompanied by chipping of some fragments is typical (Figure 9b-e). Figure 9a illustrates that the wear surface of the coating on the boundary of its fracture is quite smooth and there are signs of local chipping of coating fragments. After cutting, delaminations and longitudinal cracks also occur in the structure of the coating on the cutting tool, and the study of them on the rake face of the tool is depicted in Figure 10. The upper part of the image demonstrates a delamination between nanolayers (see Box A for a larger scale). The delamination occurs not only along the border of two nanolayer periods, but also along the border of individual nanolayers. At the same time, the delamination does not transform into a longitudinal crack, that it, does not cut the nanolayers. As noted earlier [8,30,31,35], such delaminations can reduce After cutting, delaminations and longitudinal cracks also occur in the structure of the coating on the cutting tool, and the study of them on the rake face of the tool is depicted in Figure 10. The upper part of the image demonstrates a delamination between nanolayers (see Box A for a larger scale).
The delamination occurs not only along the border of two nanolayer periods, but also along the border of individual nanolayers. At the same time, the delamination does not transform into a longitudinal crack, that it, does not cut the nanolayers. As noted earlier [8,30,31,35], such delaminations can reduce the level of internal stresses and can thus play some positive role by slowing down the process of coating fracture. The lower part of the image depicts a delamination transforming into a transverse crack (see Box B for details). The formation of such cracks may be associated with the effect of residual longitudinal compressive stresses. Another reason for the formation of delaminations and longitudinal cracks can be transverse cyclic tensile stresses formed under the influence of adhesive fatigue wear processes arising during cutting [8,30,31,35,48].
Coatings 2020, 10, x FOR PEER REVIEW 10 of 13 the level of internal stresses and can thus play some positive role by slowing down the process of coating fracture. The lower part of the image depicts a delamination transforming into a transverse crack (see Box B for details). The formation of such cracks may be associated with the effect of residual longitudinal compressive stresses. Another reason for the formation of delaminations and longitudinal cracks can be transverse cyclic tensile stresses formed under the influence of adhesive fatigue wear processes arising during cutting [8,30,31,35,48].

Conclusions
The properties of the Ti-TiN-(Ti,Cr,Mo,Al)N multilayered composite coating with a wearresistant layer of nanolayer structure were studied. The conducted studies have found the following: (1) The average coating hardness was 42 ± 1.3 GPa.
(2) The value of nanolayer period λ is about 120 nm, and the thicknesses of nanolayers are within the range of 1-8 nm.

Conclusions
The properties of the Ti-TiN-(Ti,Cr,Mo,Al)N multilayered composite coating with a wear-resistant layer of nanolayer structure were studied. The conducted studies have found the following: (1) The average coating hardness was 42 ± 1.3 GPa.
(2) The value of nanolayer period λ is about 120 nm, and the thicknesses of nanolayers are within the range of 1-8 nm.
(3) The studies of the phase composition of the coating have revealed the presence of a main cubic phase of (Ti,Nb,Zr,Al)N with Fm3m space group. Weak reflections with P6.3mc space group belong to the h-AlN phase. (4) It is found that the grain sizes in the coating under study can significantly exceed the thicknesses of its nanolayers and the value of nanolayer period λ. While nano-sized grains (5-15 nm) are detected, there are also columnar crystals 1-2 µm long. (5) After 16 min of cutting, the wear rate for the tool with the Ti-TiN-(Ti,Cr,Mo,Al)N coating was 1.9 times lower compared to the wear rate for a tool with the (Ti,Al)N commercial monolithic coating. (6) The cracking patterns in the coating on the rake and flank faces of the tool demonstrate a considerably brittle nature of fracture, accompanied by the chipping of separate fragments of the coating. At the same time, delaminations between nanolayers of the coating were also detected.